I have a simple WAI application (Warp in this case) that responds to all web requests with "Hi". I also want it to display "Said hi" on the server each time a request is processed. How do I perform IO inside my WAI response handler? Here's my application:
{-# LANGUAGE OverloadedStrings #-}
import Network.Wai
import Network.HTTP.Types (status200)
import Network.Wai.Handler.Warp (run)
main :: IO ()
main = do
putStrLn "http://localhost:3000/"
run 3000 app
app :: Application
app _ = return hello
hello = responseLBS status200 [("Content-Type", "text/plain")] "Hi"
The type of a WAI application is:
type Application = Request -> Iteratee ByteString IO Response
This means that a WAI application runs in an Iteratee monad transformer over IO, so you'll have to use liftIO to perform regular IO actions.
import Control.Monad.Trans
app _ = do
liftIO $ putStrLn "Said hi"
return hello
Related
I am following the basic dispatching section of wai application.
I am able to catch the url parameter. How can I perform IO operation using these params.
I would like to use runCommand of System.Process to execute a system command using these parameters.
:t runCommand give
runCommand :: String -> IO ProcessHandle
my Main.hs
{-# LANGUAGE OverloadedStrings #-}
module Main where
import Control.Monad
import Network.Wai
import Network.HTTP.Types
import Network.Wai.Handler.Warp (run)
import Data.ByteString
import Control.Monad
import System.Process
import qualified Data.ByteString.Lazy.Char8 as L8
main :: IO ()
main = do
run 8080 app
app :: Application
app request respond = respond $ case rawPathInfo request of
"/" -> indexHtml
"/wake" -> wakeMeUP request
_ -> fourNotFour
indexHtml :: Response
indexHtml = responseFile
status200
[("Content-Type","text/html")]
"index.html"
Nothing
wakeMeUP :: Request -> Response
wakeMeUP request =
let query = queryString request
hour = join $ lookup "hour" query
min = join $ lookup "min" query
--I would like to use runCommand "using hour and min variables"
in responseLBS
status200
[("Content-Type","text/plain")]
"Alarm set at...to be coded later"
fourNotFour :: Response
fourNotFour = responseLBS
status404
[("Content-Type","text/plain")]
"404 not found"
Your design prevents it, because of how you have written app,
app request respond = respond $ case rawPathInfo request of
which says that you immediately respond. Note the type of Application:
type Application = Request -> (Response -> IO ResponseReceived) -> IO ResponseReceived
Since the result type has an IO, you have the opportunity to do I/O before yielding a value. So you could:
app request respond = do
whateverResult <- runCommand "whatever"
respond $ ...
(You could also do it afterward, at least according to the types:
app request respond = do
rcvd <- respond $ ...
runCommand "whatever"
return rcvd
Though that's a bit odd to do for the continuation-passing idiom being used here (the (a -> b) -> b pattern in the Application type). It means that the command will be run after everything else, for some definition of "everything else" that we can't know without reading the wai source.)
Anyway, you probably don't want the command to be run inside app, but rather in wakeMeUp, which means you need to change some types around. In particular,
wakeMeUp :: Request -> IO Response
-- ^^
and suitably monadify the function. Then your app needs to not call respond immediately, so you can say
app request respond =
response <- case rawPathInfo request of
"/" -> return indexHtml
-- ^^^^^^
"/wake" -> wakeMeUp request
-- no change, because wakeMeUp now has an IO return type
...
respond response
If this is gibberish to you, it's time to do some monad tutorials. Or if you just want to get the damn thing working, I recommend Dan Piponi's The IO Monad For People who Simply Don't Care. Happy hacking!
Problem trying to unit test routes. Scotty, Persistent, and Hspec-WAI.
Unlike Yesod or Spock, Scotty doesn't have a nice place to store database handlers. I've got it working by having one massive "do" that starts up the database, keeps the database pool as a local variable, then uses that variable.
app :: IO ()
app = do
-- allocate_database $ \pool
-- scotty 8080 $do
-- handleSomeRoute pool
However, Hspec-WAI wants it in the IO Application form.
scottyApp :: ScottyM () -> IO Application
Is there a sane way to inject the DB connection pool into a scottyApp ?
Here's how you can do it. Basically you open the database before you make the hspec call:
{-# LANGUAGE OverloadedStrings #-}
import Test.Hspec
import Test.Hspec.Wai
import Network.Wai (Application)
import qualified Web.Scotty as S
allocate_db :: (Int -> IO a) -> IO a
allocate_db = undefined
handleSomeRoute :: Int -> S.ScottyM ()
handleSomeRoute = undefined
main2 :: IO ()
main2 = allocate_db $ \pool -> do
let app' = handleSomeRoute pool
hspec $ with (S.scottyApp app') $ do
describe "GET /" $ do
it "responds with 200" $ do
get "/" `shouldRespondWith` 200
I am still trying to understand how haskell syntax works. So, here's a dead simple wai/warp application.
{-# LANGUAGE OverloadedStrings #-}
import Network.Wai
import Network.HTTP.Types (status200)
import Network.Wai.Handler.Warp (run)
myApp _ respond = respond $
responseLBS status200 [("Content-Type", "text/plain")] "Hello World"
main = run 3000 myApp
If I want to print out some text into stdout with putStrLn before returning the status 200 and "Hello World" plain text, how would I implement it?
myApp has this type:
myApp :: Request -> (Response -> IO ResponseReceived) -> IO ResponseReceived
so you can add your own IO action before returning the response like this:
myApp _ respond = do putStrLn "processing request"
respond $ responseLBS status200 [("Content-Type", "text/plain")] "Hello World"
Using the websockets library in the following way
{-# LANGUAGE OverloadedStrings #-}
module Main where
import System.IO
import System.IO.Unsafe
import Network.Socket hiding (recv)
import Network.WebSockets
import Network.Socket.ByteString
import qualified Data.ByteString.Char8 as B
import Debug.Trace
import Control.Applicative
fetch :: IO B.ByteString
fetch = do
B.putStrLn "connected"
[v4] <- getAddrInfo Nothing (Just "127.0.0.1") (Just "3000")
c <- socket (addrFamily v4) Stream 0x6
c `connect` (addrAddress v4)
recv c 512000
proxy :: TextProtocol p => WebSockets p ()
proxy = sendTextData . unsafePerformIO $! fetch
app :: Request -> WebSockets Hybi00 ()
app r = acceptRequest r >> r `traceShow` proxy
main :: IO ()
main = withSocketsDo $! runServer "0.0.0.0" 4000 app
causes fetch to occur only once and all websocket clients receive the same not fresh data.
How can I do arbitrary IO with websockets?
How can I get the above example to work with fresh fetches?
I would love to hear any suggestions or complete solutions. A way of doing it without touching iteratee would be exceptionally appreciated.
The WebSockets monad is an instance of the MonadIO typeclass, so you can do arbitrary IO-operations with the liftIO function.
In this case I'm guessing you want to do
proxy = liftIO fetch >>= sendTextData
You also need to add the import
import Control.Monad.IO.Class
I have the basic "hello world" application setup using wai, and would like to use wai-handler-devel, but am unsure how to go about it and can't find any examples of it in usage on a wai project.
{-# LANGUAGE OverloadedStrings #-}
import Network.Wai
import Network.HTTP.Types
import Network.Wai.Handler.Warp (run)
import Data.ByteString.Lazy.Char8 () -- Just for an orphan instance
app :: Application
app _ = return $ responseLBS
status200
[("Content-Type", "text/plain")]
"Hello, World!"
main :: IO ()
main = do
putStrLn $ "http://localhost:8080/"
run 8080 app
What do I need to do to get wai-handler-devel working with a basic wai app?
Note:
There is a fix here ( https://gist.github.com/1499226) i f you run into issues with "wai-handler-devel: command not found"
wai-handler-devel's Hackage page says that it should be invoked from the command-line like so:
$ wai-handler-devel <port> My.App.Module myApp
and that your application's type must look like this:
myApp :: (Application -> IO ()) -> IO ()
In this case, you should define myApp as follows:
myApp :: (Application -> IO ()) -> IO ()
myApp handler = handler app
although you may want to inline app entirely:
myApp :: (Application -> IO ()) -> IO ()
myApp handler = handler $ \_ -> return $ responseLBS
status200
[("Content-Type", "text/plain")]
"Hello, World!"
The type is like this so that you can do initialisation on start-up and the like in IO. I suggest reading the SmallApp and FullApp examples from wai-handler-devel's git repository; the latter is especially helpful, as it has debug output showing the flow of the code during a reload, and shows how to integrate a long-running database connection.
The run script for the FullApp example also shows how to use wai-handler-devel programmatically, including manually specifying Hamlet template dependencies (which the wai-handler-devel command-line tool determines automatically).
You should then be able to rewrite your main as follows:
main :: IO ()
main = do
putStrLn $ "http://localhost:8080/"
myApp (run 8080)
Of course, you could just as easily pass the run function from wai-handler-fastcgi, wai-handler-scgi or even wai-handler-webkit.